1. Field of the Invention
The present invention relates to a ferritic Cr-containing steel sheet suitable for use for building facing materials, kitchen utensils, chemical plants, water tanks, automobile heat resistant members, etc. Particularly, the present invention relates to a ferritic Cr-containing steel sheet having excellent ductility, formability, and anti-ridging property, and a method of producing the same. In the present invention, the steel sheet includes a steel plate, and a steel strip.
2. Description of the Related Art
Stainless steel sheets have beautiful surfaces and excellent corrosion resistance, and are thus widely used for building facing materials, kitchen utensils, chemical plants, water tanks, etc. Particularly, austenitic stainless steel sheets have excellent ductility and excellent press-formability, and thus cause no ridging as a result of pressing, and are widely used for the above applications.
On the other hand, ferritic Cr-containing steel sheets such as ferritic stainless steel sheets need to be improved in formability. This is done by purifying the steel. The use for the above applications, instead of austenitic stainless steel, sheets of SUS 304, SUS 315, etc. have recently been studied. This is because the properties of the ferritic stainless steel are widely known, for example, low thermal expansion coefficient, low sensitivity to stress corrosion cracking, and low cost due to the absence of the expensive Ni ingredient.
However, for application to formed products, the ferritic stainless steel sheets have lower ductility than the austenitic stainless steel sheets, and this causes problems in that xe2x80x9cridgingxe2x80x9d occurs in the surfaces of the formed products. Ridging is an unevenness that spoils or downgrades the beauty of the formed products, significantly increasing the polishing load. Therefore, in order to further extend the application of ferritic stainless steel sheets, improvement in all of ductility, formability and anti-ridging properties are required.
For these requirements, ferritic stainless steel having excellent formability comprises 0.03 to 0.08 wt % of C, 0.01 wt % or less of Ni, and 2xc3x97N wt % to 0.2 wt % of Al and is proposed in, for example, Japanese Unexamined Patent Publication No. 52-24913. In the technique disclosed in Japanese Unexamined Patent Publication No. 52-24913, the C and N contents are decreased, and the Al content is twice or more as much as the N content decreasing the amount of solute nitrogen and making the crystal grains fine, thereby improving ductility, anti-ridging properties, and secondary formability.
However, in the technique disclosed in the aforesaid Publication No. 52-24913, the formability is greatly improved, but the anti-ridging properties are not significantly improved. Therefore, when working such as press forming or the like is performed, polishing is required for improving the beauty of the metal surface, increasing cost due to increased polishing load.
On the other hand, for example, Japanese Unexamined Patent Publication No. 51-123720 discloses a method for reducing ridging, in which after hot rolling, rolling is performed with a rolling reduction of 15% or more in a temperature region of 450 to 700xc2x0 C., followed by annealing, cold rolling and final annealing.
Although the technique disclosed in the aforesaid Publication No. 51-123720 improves the anti-ridging properties, the technique does not significantly improve ductility or formability. Therefore, the various further attempts have been made to improve all of ductility, formability and anti-ridging properties concurrently.
Japanese Unexamined Patent Publication No. 2-170923 discloses a method of producing a chromium stainless steel sheet having excellent anti-ridging properties and press-formability, in which a hot-rolled sheet obtained by hot-rolling a chromium stainless steel containing 13.0 to 20.0 wt % of chromium is subjected to pre-cold rolling with a rolling reduction of 2 to 30%, followed by continuous annealing, descaling, cold rolling, and finish annealing. Strain is achieved by cold rolling before annealing to promote recrystallization in annealing, permitting continuous annealing for improving formability and anti-ridging properties.
The occurrence of ridging is a fundamental problem and is inherent in a ferritic stainless steel sheet. It needs to be fully resolved. On the other hand Japanese Unexamined Patent Publications Nos. 9-263900 and 10-330887 disclose a technique for improving anti-ridging properties by controlling a colony of similarly oriented crystal grains.
Although the technique disclosed in Japanese Unexamined Patent Publication No. 2-170923 improves the so-called xe2x80x9crxe2x80x9d value (Lankford value) and the anti-ridging properties, the technique has a problem in that there is still considerable room for further improvement of both those properties, and that it fails to improve significantly the anti-ridging property and r value of the stainless steel.
Although the techniques disclosed in Japanese Unexamined Patent Publications Nos. 9-263900 and 10-330887 can prevent the occurrence of a colony of similarly oriented grains, they both face the problem that the occurrence of ridging cannot be completely suppressed, with the products exhibiting poor surface qualities after forming.
Furthermore, deeply drawing a ferritic stainless steel by press forming or the like encounters the problem of planar anisotropy of the xe2x80x9crxe2x80x9d value and elongation of the steel sheet. Even when the steel sheet has a high mean xe2x80x9crxe2x80x9d value and a mean elongation value in each direction, with a low minimum xe2x80x9crxe2x80x9d value and a minimum elongation value, deep drawing cannot be sufficiently performed. In the steel sheets produced by the above-described conventional techniques, the mean xe2x80x9crxe2x80x9d value and mean elongation are improved, while the minimum xe2x80x9crxe2x80x9d value and minimum elongation value are low enough to cause a problem of high planar anisotropy of the xe2x80x9crxe2x80x9d value and the elongation value.
The above-described conventional techniques cannot produce a ferritic stainless steel sheet satisfying the need for better ductility, formability, and anti-ridging properties at low cost. Namely, in the conventional techniques, formability is greatly improved, while the effect of improving the anti-ridging property is insufficient. Therefore, in an application using working such as press forming or the like an increased polishing load is necessary for improving the surface beauty of the formed product. In addition, although the mean xe2x80x9crxe2x80x9d value and mean elongation value are improved, the problem remains that sufficient formability cannot be obtained in actual press forming (or the like) because of the high planar anisotropy of the xe2x80x9crxe2x80x9d value and elongation, thereby causing difficulties in producing steel having sufficient levels of ductility, formability and anti-ridging properties at low cost.
The present invention has been achieved for solving the problems associated with the above-described conventional techniques.
An object of the present invention is to provide a ferritic Cr-containing steel sheet having good ductility and formability, while also having excellent anti-ridging properties, particularly an anti-ridging property equivalent to that of stainless steel SUS304, and excellent surface qualities after forming, and a method of producing the same. Another object of the present invention is to provide a ferritic Cr-containing steel sheet having good ductility and formability, excellent anti-ridging properties, and low planar anisotropy of the xe2x80x9crxe2x80x9d value, along with excellent elongation characteristics.
This invention also relates to a method of producing such a ferritic Cr-containing steel sheet.
We have discovered the importance of specific chemical components and proportions in the steel, and the steps of pre-rolling performed by warm or cold rolling with a relatively low rolling reduction between hot-rolling and hot-rolled sheet annealing to improve ductility, formability and the anti-ridging property. We have further found that in combination with these steps, about 0.0002 to 0.0030% of B can be added to significantly decrease the planar anisotropy of elongation of the steel. We have further found that the finishing delivery temperature FDT of the hot rolling step shall be set to a value as low as 850xc2x0 C. or less, and that this increases the minimum xe2x80x9crxe2x80x9d value rmin and significantly improves the planar anisotropy of the xe2x80x9crxe2x80x9d value, leading to the achievement of the remarkable qualities of the steel of the present invention.
In the present invention, the hot-rolled sheet annealing step may comprise either box annealing or continuous annealing. However, in continuous annealing, a stabilizing element such as Ti or Nb must be added to the steel in which the C and N contents are decreased, and B is added to the steel in amounts more fully described hereinafter.
In the present invention, as a result of studies of a basic solution of the fundamental ridging problem in a ferritic Cr-containing steel sheet, with special attention to the crystal grain structure of the steel sheet, it was found that the anti-ridging properties are significantly improved by decreasing the elongation index of the steel. This is defined as the ratio of the length of the crystal grains in the rolling direction to the length of the crystal grains in the thickness direction after hot-rolled sheet annealing. It was also found that the occurrence of ridging can be significantly suppressed by suppressing the formation of a colony comprising coarse crystal grains generally oriented in the rolling direction of the cold-rolled annealed steel sheet. This is an important achievement of the present invention.
The present invention provides a method of producing a ferritic Cr-containing steel sheet comprising the step of hot-rolling a steel raw material comprising about 0.001 to 0.12% of C, about 0.001 to 0.12% of N, and about 9 to 32% of Cr, all percentages herein being mass percentages. After the hot-rolled sheet annealing step, a cold-rolling step comprises cold-rolling the hot-rolled sheet passed through the hot-rolled sheet annealing step to form a cold-rolled sheet, followed by the finish annealing step.
In the method, a pre-rolling step is performed by cold or warm rolling at a rolling reduction of about 2 to 15% between the hot-rolling step and the hot-rolled sheet annealing step. In the present invention, the hot-rolled sheet annealing step uses a Cr-containing steel raw material comprising components appropriately controlled for box annealing or continuous annealing.
In box annealing, in the hot-rolled sheet annealing step, the hot-tolled sheet is preferably maintained at a predetermined annealing temperature for about 1 hour or more, and cooled to about 600xc2x0 C. at a mean cooling rate of less than about 25xc2x0 C./h after retention, and the annealing temperature is more preferably in the range between the about (A1 transformation point +30)xc2x0 C. to about 1000xc2x0 C. In the practice of the present invention, in order to decrease the planar anisotropy of the xe2x80x9crxe2x80x9d value, and the elongation, the finishing delivery temperature in the hot rolling step is preferably controlled at about 850xc2x0 C. or less, and about 0.0002 to 0.0030% of B is preferably added.
The product of the present invention comprises a ferritic Cr-containing steel sheet having excellent ductility, formability and anti-ridging properties, and comprises, in mass %, about 0.001 to 0.12% of C, about 0.001 to 0.12% of N, and about 9 to 32% of Cr, and comprises a section of the steel sheet in the thickness direction parallel to the rolling direction, having an elongation index of crystal grains of about 5 or less at any position.
The present invention also provides a method of producing a ferritic Cr-containing cold-rolled annealed steel sheet having excellent ductility, formability, and anti-ridging properties, comprising the steps of cold rolling the steel sheet to the extent of about 30% or more, and finish annealing at about 700xc2x0 C. or more.
The present invention further provides a ferritic Cr-containing steel sheet having excellent ductility, formability, and anti-ridging properties, and comprises, by mass %, about 0.001 to 0.12% of C, about 0.001 to 0.12% of N, and about 9 to 32% of Cr, wherein the steel sheet has a crystal grain structure in which in a section of the steel sheet in the thickness direction parallel to the rolling direction, a coarse grain colony of crystal grains having a crystal grain area larger than about 2xc3x97A0, which A0 designates the mean crystal grain area, and oriented in the rolling direction has an aspect ratio of about 5 or less.